1. Field of the Invention
The present invention relates to the known field of bottom-to-surface links of the type comprising a vertical submarine pipe known as a xe2x80x9criserxe2x80x9d connecting the sea bed to a floating support located on the surface.
2. Description of the Prior Art
Once the depth of water becomes large, production fields, and in particular oil fields, are generally operated from floating supports. In general, such a floating support has anchor means to keep it in position in spite of the effects of currents, winds, and swell. Such a support generally also has means for storing and processing oil, and means for off-loading oil to off-loading tankers. Such tankers arrive at regular intervals to off-load production. Such floating supports are referred to by the initials FPSO (for floating, production, storage, off-loading). Numerous variants have been developed such as SPARS which are long floating cigar shapes held in position by catenary anchor systems, or indeed TLPs, i.e. tension leg platforms, in which the tension legs are generally vertical.
Well heads are often distributed over an entire field, and production pipes together with pipes for injecting water and cables for monitoring and control are placed on the sea bed to converge on a fixed installation with the floating support being positioned on the surface vertically above said installation.
Some wells are situated vertically beneath the floating support and the insides of such wells are. therefore directly accessible from the surface. Under such circumstances, a well head fitted with its xe2x80x9cChristmas treexe2x80x9d can be installed at the surface on board the floating support. It is then possible using a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required by a well throughout the lifetime of said well. This is referred to as a xe2x80x9cdryxe2x80x9d well head.
To keep the riser fitted with its dry well head in a substantially vertical position, it is appropriate to exert upwardly directed traction near its top end, which traction can be applied either by a cable-based tensioning system using winches or hydraulic actuators installed on the floating support, or else by means of floats distributed along the riser and installed at various depths, or else by a combination of those two techniques.
U.S. Pat. No. 2,754,011 (IFP) discloses a barge and a guide system for a riser, the riser being fitted with floats.
SPARS and TLPs are also fitted with risers tensioned by floats.
When floats are used for tensioning purposes, it is necessary:
either to make syntactic foam elements that are installed as a half-shell around the riser;
or else to make floats out of metal or composite material, which floats are filled with gas, preferably an inert gas such as nitrogen.
Syntactic foam is a foam containing microbeads of glass impregnated in an epoxy or polyurethane type resin. This type of foam has exceptional ability to withstand pressure and it is commonly used at great depths. However this type of foam suffers from the drawback of being very expensive and difficult to make.
Since the depth of water over some oil fields exceeds 1500 meters (m), and can be as great as 2000 m to 3000 m, the weight of a riser over such a depth requires forces that can be as much as or greater than several hundreds of (metric) tonnes in order to enable them to be kept in position. For such extreme depths (1000 m-3000 m), buoyancy elements of the xe2x80x9ccanxe2x80x9d type are used which are installed on the risers at various depths. The floats concerned are then of large dimensions, and in particular they can be of a diameter lying in the range 1.5 m to 5 m, or even greater than 5 m, and they can extend over a length of 10 m to 20 m in order to achieve buoyancy of as much as 100 tonnes.
The float and the pipe are subject to the effects of swell and of current, and because they are connected to an FPSO on the surface, they are also subjected indirectly to the effects of wind. This gives rise to considerable lateral and vertical movements (several meters) in the system comprising the riser, the floats, and the barge, particularly in the zone thereof that is subject to swell. These movements give rise to large differential forces between the riser and the float. In addition, the curvature taken up by the riser can give rise to very large bending moments in the change of second movement of area that arises where there is a connection between the riser and a float.
In order to minimize the forces generated by current and swell acting on the riser-and-float system, floats are generally circular and they are installed coaxially around the riser.
In addition, floats are generally fixed to a riser in such a manner as to ensure that the connection between a riser and a float is leakproof and capable of confining the filler gas within the float. The commonly-employed solution consists in mutual interfitting engagement between the float and the riser at the top and bottom ends of the float, backed up by welding. Numerous reinforcements are added to ensure that the assembly has sufficient strength.
At such a junction between a riser and a float, the second moment of area of the assembly (i.e. its resistance to bending) varies considerably on going from the section of the riser to the section of the float.
Such large variations in second moment of area give rise to poor stress distribution, thus giving rise to highly localized zones where stress can become unacceptable and can lead either to sudden breakage or else to fatigue, leading in turn to the appearance of cracks and then to ruin. In order to reinforce the sensitive zone, these localized stresses require transition pieces to be used, generally large conical pieces referred to as xe2x80x9ctapered jointsxe2x80x9d. In some cases, these pieces can be as much as 10 m long, and in the best of cases they require very high performance steels to be used. However, it is often necessary to use titanium which is about five to ten times more expensive than the best steels. Furthermore, these pieces are generally complex in shape and they must be made to extremely high standards in order to guarantee the service expected of them over the lifetime of such equipment which commonly exceeds 25 years.
U.S. Pat. Nos. 3,952,526 and 3,981,357 disclose junction systems between floating tanks and risers, where parts are used that are made of elastomer material.
Those buoyancy systems enable the tensioning system on board the floating support to be reduced and, in general, they are distributed over a large fraction of the depth of water, and in addition they present small buoyancy of up to a few hundreds of kg, or perhaps as much as 1 tonne or 2 tonnes.
The junctions are situated in the top portions of the floats, while the bottom portions of the floats are generally left open. Such devices can transfer loads corresponding to reducing the weight of a limited length of pipe, but they are not suitable for floats that are intended (on their own and without help from additional tensioning systems secured to the floating support) to support very long lengths of riser, e.g. 500 m to 1000 m, or even more, of the kind to be found in off-shore oil fields at greatdepth, i.e., in particular, at depths of more than 1000 m. The buoyancy required for providing tensioning by means of floats alone requires considerable forces to be transferred vertically and transversely, said vertical forces applied to the head of the riser being capable of reaching several hundreds of tonnes, and in particular lying in the range 300 tonnes to 500 tonnes.
The object of the present invention is to provide a novel type of junction between a riser and a can so as to enable large loads to be supported and transferred while mitigating the drawbacks of the above-described floats assembled around said pipe by mutual engagement.
An object of the present invention is thus to provide a novel riser-and-float junction means that is simple, flexible, and reliable, mechanically speaking, and in particular that serves to reduce phenomena of fatigue and wear due to the stresses which act at the junction which is subjected to loads of several hundreds of tonnes.
Another object of the present invention is to provide a riser-and-float junction for a riser that is to be used in great depths, and that is essentially, or even solely, tensioned by floats of large dimensions.
For this purpose, the present invention provides a bottom-to-surface connection system comprising a submarine pipe assembled to at least one float having a coaxial can surrounding said pipe and junction means for joining said can to said pipe at top and bottom orifices of said can through which said pipe passes, the system being characterized in that:
said junction means comprise a resilient and leakproof flexible joint assembled around said pipe at at least one of the top and bottom orifices of said can;
said resilient and leakproof flexible joint includes a laminated abutment comprising a first plate secured to said pipe or to said can, and a second plate secured to said can or to said pipe, respectively, said plates sandwiching layers of elastomer and rigid reinforcement, said plates, elastomer layers, and reinforcement presenting surfaces of revolution about an axis constituted by the axis of said pipe and said duct when at rest; and
said laminated abutment being:
a frustoconical plane laminated abutment constituted by a plurality of layers of elastomer and rigid reinforcement in the form of superposed washers, including a rigid plate of frustoconical shape inscribed within a frustoconical envelope surface, said frustoconical plate surmounting said plurality of superposed layers of elastomer and rigid reinforcement;
or else a cylindrical laminated abutment constituted by a plurality of layers of elastomer and rigid reinforcement in the form of adjacent coaxial tubes.
In said frustoconical or cylindrical laminated abutments, said layers of elastomer and rigid reinforcement can be continuous or discontinuous, and they are preferably continuous. The term xe2x80x9cdiscontinuousxe2x80x9d is used herein to mean that a layer is made up of a plurality of portions defining circular sectors which together make up one of said washers, or one of said frustoconical or cylindrical surfaces of revolution.
In another embodiment, said junction means comprise a flexible, resilient, and leakproof joint at each orifice of said can.
In another embodiment, said junction means comprise a leakproof joint at one of the orifices of said can, and in particular its top orifice, and a joint that is flexible, resilient, and leakproof at its other orifice.
More particularly, one of the top and bottom orifices of said can has a resilient, flexible, and leakproof joint constituted by a said cylindrical laminated abutment, and the other orifice has an abutment comprising a rigid plate of frustoconical shape with its larger base situated at its bottom end, and said plate preferably surmounts a said laminated abutment.
A flexible, resilient, and leakproof joint of the present invention transmits stresses at a junction:
in twisting movement of said pipe relative at least to axes perpendicular to the axis of said can such that the axis of said pipe can depart angularly from the axis of said can at said orifices, with the angle between the axis of said pipe and the axis of said can at one of said orifices preferably varying over a range of 0 to 10xc2x0, and more particularly over a range of 0 to 5xc2x0; and
in movement(s) of said pipe including a component in translation transversely to said can in directions perpendicular to the axis of said can such that the axis of said pipe can depart transversely from the axis of said can within said orifices, and more particularly the axis of said pipe can depart from the axis of said can by 0 to 5 cm.
Advantageously, said frustoconical plate has a central cylindrical cavity that is secured to said pipe, and said layers of elastomer and rigid reinforcement in the form of superposed washers are inscribed with a frustoconical envelope surface.
In an advantageous embodiment, said frustoconical plate is an integral portion of the riser.
The conical shape in accordance with the present invention contributes to transmitting vertical forces to said pipe in a manner which is progressive. The vertical thrust that acts on the can is transmitted via the laminated abutment in the direction of the axis of said pipe. Said laminated abutment presents much lower stiffness in directions that are perpendicular thereto, said transverse stiffness being adjustable as a function of the quality of the elastomer, the thickness of the various layers of elastomer, and the number of such layers.
More particularly, the cylindrical laminated abutment has an inner first rigid cylindrical plate secured to the outside surface of said pipe and an outer second rigid cylindrical plate secured to said can, preferably via an annular flange.
The cylindrical shape of the laminated abutment of the invention confers low stiffness to said abutment along the axis of said can and of said pipe, and transmits practically no force in this vertical direction. In contrast, this cylindrical shape presents stiffness in directions perpendicular to thereto which favors the transfer of load between the can and the pipe while also allowing twisting about the transverse axes, and to a smaller extent about the vertical axis.
The combination of a frustoconical plane laminated abutment and a cylindrical laminated abutment favors all twisting and translation movement at the orifices as mentioned above, said twisting movements being associated with said pipe curving within said can.
The buoyancy of said submarine pipe is preferably provided by said floats without adding additional tensioning systems, in particular without systems operating by means of cables that are secured to the floating support.